This invention relates generally to the use of optics for testing semiconductors and integrated circuits for defects, and more particularly the preferred embodiment of this invention relates to use of a video microscope assembly for emission microscopy comprised of an objective lens, optic tube, video imaging system (CCD or similar imager) and a microscope movement on a vibrating integrated Circuit Test Head (ATE, Automatic Test and Evaluation) test head.
Light emission microscopy is used to detect defects in integrated circuits. See U.S. Pat. Nos. 4,680,635, 4,755,874, 4,811,090 and 5,475,316 for example. Briefly an emission microscope operates on the principle of detection of recombinant radiation. In an excess current drawing condition electron hole and pairs recombine and relax giving off cold photon emissions which can be detected using specialized CCD based sensors. The procedure is performed on wafers and on decapsulated (mold compound removed) or de-lidded ceramic packages viewing the die traditionally from the frontside but also through the backside of the silicon.
The procedures primary function is failure localization. The test can be run as simply as providing power and ground to the chip to evaluate standby leakage. The preferred embodiment of this invention particularly relates to the use of ATE test heads to send streams of test vectors to exercise the chip and bring it into a failing logic state. But any vibrating environment could apply. Without the capability of dynamically sending test vectors to xe2x80x9cbring onxe2x80x9d the defect state via the ATE head, the chip will not fail. Inherent to this operation is the use of ATE test heads, which when running are cooled with fans or liquids and run often on raised computer floors. In the use of an emission microscope on a test head magnifications to 1500xc3x97 are produced by bringing objective lenses close to the chip to collect light and pass it on the CCD detector. The vibration inherent on test heads limits significantly the overall magnification achievable due to optical degradation.
A portable emission microscope was described in U.S. Pat. No. 5,475,316 for example which is used by conjunction with ATE test heads but separate from the ATE head when operating through the use of an offset. Means must be provided to for damping or coupling vibrations to eliminate distortion due to relative motion between the microscope and the test head.
U.S. Pat. No. 5,764,409 discloses the use of interlocking cylinders that telescope in front of the objective lens and couple the vibration of the ATE head into the cylinder by means of simple friction thereby reducing vibration between microscope and test head. Shown in FIG. 1, the microscope 301 is threadably coupled through collar 401 to cylinder 406 which in turn threadably engages a second cylinder 403 which contacts test head surface 408 in pressure attachment through simple friction.
Copending application Ser. No. 09/248,783, supra, is directed to couple optics (optical tube and or objective lenses) in use on vibrating ATE test heads thereby eliminating relative motion between the two with the net effect of eliminating optical distortion in the CCD or sensor image plane. The preferred embodiment also simplifies focusing and provides greater access to the region within the field of view of the optic for microprobing or exchange of the devices from within test sockets. Further, to any one skilled in the art of semiconductor analysis, access to the devices under the field of view of the optics is essential for microprobing individual lines, junctions or bond pads. Microprobing with needles is achievable through the application of the preferred embodiment of this invention and is an improvement over the prior art.
The present invention is an improved embodiment of the invention described in copending application Ser. No. 09/248,783.
Briefly, in accordance with light emission microscope already established, the objective lenses must be within its working distance (typically 12-75 mm) of the device under test (DUT) semiconductor. The DUT is on top of a vibrating ATE test head, in a socket, or in wafer form; is on a chuck in a wafer sorter docked to the ATE head. In all case vibration is present through the floor to the tester and by fans and cooling within the test head itself. The objective and the optical tube to which they are attached are coupled to a motorized drive to allow for X and Y positioning and Z axis control of focus. This assembly is referred to as the microscope movement. The movement itself is rigidly attached to a solid mechanism that fully supports the static load of the optics and microscope movement. This rigid mechanisms provides an undesirable path for transferring vibration (either test floor, test head or both) into the objective lens through the path described above.
A vibration coupling assembly is provided which is dynamic in nature and which operates under computer control. This mechanism is composed of a rigid assembly which provides on one side, direct coupling to the microscope and secondly, direct coupling to the test head when actuated. This rigid assembly and its points of engagement provide a vibration damping coupling of the microscope to the test head.
In a preferred embodiment, the vibration coupling assembly includes a plurality of piston driven rods coupled to the microscope with the rods engaging the test head when the pistons are actuated. The pistons can be driven pneumatically, hydraulically, or by solenoid, for example. A mount is provided for attaching the plurality of piston driven rods to the microscope, and the mount can include a plurality of pivotal arms on which the plurality of piston driven rods are attached whereby the plurality of piston driven rods can have variable spacing when engaging the test head.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.